1. Field of the Invention
The present invention relates to a technique of generating halftone dots in response to an image signal, and more particularly, to improvement of tone reproduction of halftone images.
2. Description of the Related Art
Binary dot signals representing halftone dots are typically obtained by comparing an image signal with a threshold pattern. The threshold pattern may also be referred to as `threshold matrix`. A threshold pattern is generally prepared to cover an area in which a single halftone dot is to be formed. In this case, the number of reproducible tones in a halftone image is smaller by one than the number of pixels included in the threshold pattern at the maximum. This is also smaller by one than the number of pixels included in a one-dot area in which a single halftone dot is to be formed.
Recently there has been a strong demand for high-speed output processing of images, and thus for production of halftone dots of a certain screen ruling at a lower resolution. This leads to reduction of the number of pixels included in a one-dot area. Reduction of the number of pixels included in a one-dot area, however, undesirably lowers the number of tones reproducible by one halftone dot.